Vacuum tube



Patented Jan. 5, 1932 LEON HCUULLOCH, F PITTSBURGH, PENNSYLVANIA,ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATIONOF PENNSYLVANIA VACUUM TUBE Ho Drawing.

. My-invention relates to electrical discharge devices and moreparticularly to thermionic ubes and photoelectric tubes.

An object of my invention is to obtain upon a desired surface, ametallic electron emitting film.

Another object is to introduce into electrical discharge devices certainmetals which oxi- 'dize easily and without the use of complicatedmachinery and involved processes.

Still another object is to provide an ellicient getter for the purposeof removing residual gases during vacuum tube manufac ture.

A further object is to decrease the cost of production of such devicesas thermionic tubes, photoelectric tubes, and other analogous devices inwhich are used electronically active metals.

2 It is well known that the alkaline earth metals may be introduced intovacuum tubes by means of distillation. This is described and illustratedin the patent to Kunz No. 1,381,474, June 14, 1921. The processdescribed in the patentinvolves considerable technical skill if asatisfactory product is to be obtained. Another method of introducing adesired metal into the vacuum tube is illustrated and "described in thepatent to Langmuir, 1,273,628,=-dated July 23, 1918, in which a filamentof the metal is vaporized in the interior of the tube. Both of thesemethods will result in tubes having photoelectric or electronicallyactive films deposited in desired locations but in their practicalapplication they are accompanied by certain well knowndifiiculties.

Certain metals which are desirable as coatings for tube elements, etc.are so readily oxidizable that it is very difficult to introduce themsatisfactorily into tubes. The most notable of such metals is caesium,which is extremely desirable from a thermionic or photoelectricstand-point. It is also diflicult to properly introduce barium in theusual and well known ways. 7

By means of my invention, I am enabled to seal in a tubeany of thealkali and alkali earth metals including, for example, potassium,sodium, rubidium, caesium, barium,

Application filed ma 20,

1926. Serial No. 110,547.

and possibly calcium and strontium Without any special difiiculty.

I make use of chemical reaction involving the decompositionofhydronitric acid salts of the desired metals directly at the point wherethe metals themselves are desired. In this way I am enabled to performthe whole operation Within the vacuum tube device or directly upon theelectrode without exposing the resulting metal to the atmosphere. I amalso enabled to deposit within a tube, a metallic getter which will tendto remove certain residual gases.

In practicing my method, the trinitride of the desired metal is placedin a receptacle (5 or vacuum tube or on an electrode thereof and thetube is then baked at a temperature not exceeding 200 C;, at the sametime subjecting it to a preliminary exhaust. The temperature of 200might in some cases be exceeded if the characteristics of the salt usedpermitted. It should be observed that thc preliminary heating must bewell below the point of incipient decomposition of the salt as it isextremely important that this salt be 7 kept from decomposing until suchtime as the preliminary exhaust is completed.

Having exhausted the tube or other device to a pressure oflO m. m. ofmercury, I next bake it at a temperature high enough to de- 0 composethe trinitride into the metal and nitrogen. This second heating Willvary from 200 to 500 C. depending upon the trinitride employed.

The next step is the final exhaust to carry off the nitrogen which, ofcourse, is not desired in the tube. It is obvious that the first bakingand exhausting may be performed simultaneously as well as the secondcorresponding operations, the only precaution necessary being that thedecomposition temperature must not be reached until the preliminaryexhaust is complete so that there will be no danger of oxidation of theresulting metal. It is thus apparent that, by means of this method, Ican either coat the walls of the receptacles or can place a deposit of athermionic or photoelectric metal on a desired electrode.

It is also believed obvious that the deposit may be first placed on oneelectrode and then by applying suitable heat by suitable means it ma bedriven across to another electrode.

This method of forming a deposit directly at the point at which it is tobe utilized without subjecting such deposit to the action of theatmosphere is novel inthis art. I have found that barium, sodium,potassium, rubidium. and caesium trinitrides respond quite readily tothis method. Lithium, calcium and strontium trinitrides decompose asreadily but appear to break down into lower nitrides.

To lower the temperature of decomposition of caesium trinitride, I havefound it best to mix equal parts of barium trinitride and caesiumtrinitride. When this mixture is heated, the caesium distils out at atemperature below the softening point of soft glass. This mixture is,however, ex losive to a certain extent and I find that y mixing with ita certain proportion of inert material such as zirconium silicate orsilicon dioxide (SiO the barium caesium trinitride mixture decomposeseasily and quickly and the danger of explosion is entirely obviated. Asan alternative method there maybe used a porous mass of quartz or othercoherent material through the body of which the barium-caesiumtrinitride mixture is distributed. This mass may be mechanically held inplace'with in the receptacle and the Whole device heated as explainedabove.

When it is desired to provide a getterl in the process of tubemanufacture, thetrini-Q tride of the desired metal is introduced intothe tube and the same method is followed, resulting in the metal beingdeposited where it is desired. This metal, being in a fine state ofdivision, may absorb gases directly, or, upon subsequent heating at amuch higher temperature, this metal may be flashed with the resultingabsorption of certain residual gases. Among the metals best adapted forthis latter purpose are, sodium, barium, strontium, and calcium.

It will thus be seen thatmy invention provides a novel method ofintroducing an ox-:= idizable metal into a tube or analogous de-L vicewhere it is to be either utilized for V thermionic emission, forphotoelectric purposes, or as a getter.

Among the principal advantages of my invention is the fact that its usegreatly lowers the cost of production of electrical discharge devices ofthe character described.

Although I have described several specific embodiments of my invention,other equivalents will suggest themselves to those skilled in the artand it 'is not intended that I be bound by any limitation-other thanthat imposed by the prior art and embodied in the following claims. 7 V

I claimas my invention: 1. In a method of providing athermionically-emissive cathode, the step of heating a mixture ofcaesium trinitride and barium trinitride.

2. In a. method of providing a thermionically-emissive cathode, the stepof heating a mixture of caesium trinitride, barium trinitri'd'e and achemically inert material.

3. In amethod of providing a thermionically-emissive cathode, the stepof heating a mixture of caesium trinitride, barium trinitride' and agranular refractory.

4.'The method of providing a deposit of caesium in a vacuum tube whichincludes the steps of inserting a mixture of caesium trinitri e andbarium trinitride in said tube, evacuating said tube andheating saidmixture.

5. The method of providin a deposit-of caesium in a vacuum tube whicincludes the steps of inserting a. mixture of caesium trinitride, bariumtrinitride and a granular refractory in said tube, evacuating said tube,and heating said mixture.

7 In testimony whereof, I have hereunto subscribed my name this llth dayof May, 1926.

LEON .McCULLOCH.

